Power generators that use renewable energy, typically solar power, and systems that store inexpensive late-night power in a power storage device for the electrical energy stored therein to be consumed as a residential load during the day are becoming more common of late.
However, when output power from the power generator flows backward to the power grid, the voltage of the power grid is increased. It is thus required to prevent this increase in the voltage of the power grid by decreasing the output power from the power generator and thus decreasing the power flowing backward to the power grid when the voltage of the power grid exceeds a threshold or when a power company demands prevention of reverse power flow to their power grid.
In the case where the power generator uses renewable energy to generate electricity, a decrease in the output power from the power generator lowers the usage efficiency of the renewable energy.
Patent Literature 1 describes a conventional technology wherein, in a power management system that is connected to a power grid and includes a power generator and a power storage device, the power storage device starts being charged when the voltage of the power grid exceeds a threshold. According to Patent Literature 1, an increase in the voltage of the power grid can be prevented by decreasing the power flowing backward to the power grid but without decreasing the output power from the power generator.